powerpc/kernel: Fix ppc_save_regs inclusion in build

Recent patch fixed an old commit
'fc2a5a61 ("powerpc/64s: ppc_save_regs is now needed for all 64s builds")'
which is to include building of ppc_save_reg.c only when XMON
and KEXEC_CORE and PPC_BOOK3S are enabled. This was valid, since
ppc_save_regs was called only in replay_system_reset() of old
irq.c which was under BOOK3S.

But there has been multiple refactoring of irq.c and have
added call to ppc_save_regs() from __replay_soft_interrupts
-> replay_soft_interrupts which is part of irq_64.c included
under CONFIG_PPC64. And since ppc_save_regs is called in
CRASH_DUMP path as part of crash_setup_regs in kexec.h,
CONFIG_PPC32 also needs it.

So with this recent patch which enabled the building of
ppc_save_regs.c caused a build break when none of these
(XMON, KEXEC_CORE, BOOK3S) where enabled as part of config.
Patch to enable building of ppc_save_regs.c by defaults.

Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250511041111.841158-1-maddy@linux.ibm.com